One of seven Saudis due to be put to death on Tuesday by crucifixion and firing squad for armed robbery, speaking over a smuggled mobile phone from his prison cell, has appealed for help to stop the executions.

Nasser al-Qahtani told Associated Press from Abha general prison on Monday that he was arrested as part of 23-member ring that stole from jewellery stores in 2004 and 2005. He said they had been tortured to confess and had no access to lawyers.

"I killed no one. I didn't have weapons while robbing the store, but the police tortured me, beat me up and threatened to assault my mother to extract confessions that I had a weapon with me while I was only 15," he said. "We don't deserve death."

A leading human rights group added its appeal to Saudi authorities to stop the executions.

Qahtani, 24, said he and most of the ring were juveniles at the time of the thefts. They were arrested in 2006. The seven received death sentences in 2009, the Saudi newspaper Okaz reported.

Last Saturday, Qahtani said, Saudi King Abdullah ratified the death sentences and sent them to Abha. Authorities set Tuesday for the executions. They also determined the methods.

The main defendant, Sarhan al-Mashayeh, will be crucified for three days. The others will face firing squads.

Qahtani faced a judge three times during eight years in detention. He said the judge did not assign a lawyer to defend them and did not listen to complaints of torture.

"We showed him the marks of torture and beating, but he didn't listen," he said. "I am talking to you now and my relatives are telling me that the soil is prepared for our executions tomorrow," he said.

Saudi Arabia follows a strict interpretation of Islamic sharia law under which people convicted of murder, rape or armed robbery can be executed, usually by sword.

Several people were reported to have been crucified in Saudi Arabia last year. Human rights groups have condemned crucifixions in the past, including cases in which people are beheaded and then crucified. In 2009, Amnesty International condemned such an execution as "the ultimate form of cruel, inhuman and degrading punishment".

Abha is located deep in the south-western province of Asir. Southerners face systematic discrimination and people there are perceived as second-class citizens compared with those in the most powerful central region, where the capital and Saudi Arabia's holy shrines of Mecca and Medina are located. Political analyst Mohammed al-Qahtani said the central region gets the best services and treatment.

"The verdict is very harsh, given all the circumstances of detention and trial with no access to lawyers, but part of the problem is selectivity," he said. "If one person belonged to political heavyweight regions, the verdict wouldn't have been harsh," he added. "The south is marginalised."

He said no minister in the Saudi government, current or past, came from the south. He said he was born in the south, did not know the family of the man who talked to AP, but was familiar with the case.

The Washington-based Institute of Gulf Affairs, which is campaigning for suspension of the executions, said in a statement addressed to the UN high commissioner for human rights, that "among the reasons for the execution is that they hail from the south, a region that is heavily marginalised by the Saudi monarchy, which views them as lower class citizens".

Ali Al-Ahmed, the head of the institute, said that in Saudi Arabia, people refer to the south as "07, which is derogatory, since it refers to the last area code phone number" in the kingdom.

"The south is very poor, and that is why rebellion comes from there," he said, "and this is why sentences are harsh, because Saudi authorities want to scare them."

Human Rights Watch in a statement on Monday appealed to Abdullah to halt the executions. It said there was "strong evidence" that the trials of all seven men violated basic principles of rights to a fair trial.

"It will be outrageous if the Saudi authorities go ahead with these executions," said Eric Goldstein, deputy Middle East director at Human Rights Watch. "It is high time for the Saudis to stop executing child offenders and start observing their obligations under international human rights law."